The general objective of the proposed research is to develop a unified theory of the evolution of intraspecific interactions built upon the axioms of population genetics. Most human diseases, pathogens and parasites are transmitted during social interactions. The theory to be developed studies the effects of these interactions on the biological fitness of the individuals and predicts how these effects have evolved. Five specific projects are proposed all relating to this general theme. First, since many social groups are small and centered around the family or more extended kinship ties (this is most certainly true of the recent evolutionary past of the human species), the effects of finite population size on the evolution of interactions is to be studied in family-structured populations. A variety of techniques will be used including computer simulation and analytic analysis of a stochastic model. Second, a methodology is proposed to study the evolution of specific genetic systems which are conducive to certain social interactions. Third, since the most common interaction is most species is mating between the sexes, the evolution of sexual dimorphism is to be studied by techniques based on quantitative genetics and the theory of sexual selection. Fourth, it is proposed that a more unified theory of social evolution can be constructed around the idea of "behavioral structure". The idea of behavioral structure encorporates both interactions in kin-structured populations and interactions among individuals that can modify their activities through learning. Both learning and kinship are especially relevant to the evolution of interactions between humans. Finally, I propose to study the evolution of genetic constraints which prevent populations from attaining the states predicted by game theory. Game theory is commonly used to study evolutionary problems but there have been few analyses of the sensitivity of its predictions to the complications arising from sexual reproduction.